Described herein are marking materials, for example inks and toners, including liquid and solid (phase change or hot melt) inks, that contain birefringent nanoparticles therein. The inks may be used in a number of copying and printing devices for forming images on an image receiving substrate such as paper, plastic and the like.
The inks and toners described herein offer the advantage of enabling images formed using the inks and toners to include a unique security feature. For example, images formed with the inks and toners herein appear normally to a viewer under most viewing conditions, but unlike typical inks and toners, are also able to appear when placed between crossed polarizers. The inks can thus be used to create a security feature in documents, plastic cards, and the like, wherein the authenticity of the item can be verified by placement of the image therein or thereon between crossed polarizers; images such as forgeries formed with a different ink or toner would not appear under the crossed polarizers, thereby exposing the counterfeit nature of the item.